The object of the invention is a process for reducing the pressure for injecting a solution of water-soluble polymer into oil wells in a given field as a function of their fracturing pressure and to do this without shear in said solution.
In many oil industry operations, fluid pressure has to be reduced in a differentiated way between several injection lines.
This operation is done with chokes that are in fact variable-passage valves, regulated or not, which reduce the pressure, limiting downstream flow rates and pressures. This is particularly important for water flooding operations, where a given line serves various injection wells. Since each well has a different fracturing pressure, starting from a given pump the pressure has to be reduced and controlled precisely for each well. This equipment has become more and more sophisticated, especially for offshore operations from platforms or FPSO. They are operated remotely, electrically or pneumatically, and opened progressively as a function of the angle of rotation.
Additionally, enhanced oil recovery has been developed since the first oil crisis in 1973, especially in the USA, to increase recovery of the oil in place. The most effective method for low-temperature fields (<120° C.) is injecting a viscous solution that homogenizes the mobility of the water and oil and delivers both better flushing and increased volume in the production area.
Three methods are particularly used:                A polymer is injected at a concentration of 500 to 3,000 ppm (typically 1,000 to 1,500 ppm) with viscosities varying with water salinity from 5 to 100 cps.        A polymer and a surfactant are injected (SP), the polymer being at the same concentrations as above, where the surfactant improves the oil in place mobilization.        A polymer, a surfactant and an alkaline product are injected (ASP). The surfactant and the alkali disperse the oil into the water injected, delivering better extraction. The polymer is injected at higher concentrations (2,000-3,000 ppm) but its viscosity is reduced by the presence of alkalis at similar values (5 to 100 cpm).        
High-molecular weight water-soluble polymers are subject to mechanical degradation. It breaks the chains and reduces the viscosity of the solution injected. This reduction in viscosity can occur in any system containing shear: pumps (especially centrifuge pumps), piping, and injectors, but especially in chokes.
It is therefore necessary to find a solution that reduces shear and improves polymer efficacy.
In small oil wells, one of the solutions is to use a length of tube with smaller diameter than the pumping piping that increases the rate of passage and therefore progressively reduces pressure. Potential rates up to 10 meters per second have little effect on the mechanical degradation but long lengths of piping are needed to obtain limited pressure drop: from 50 to 200 meters on installations existing for 10 bars of pressure drop. What is more, it is necessary to change the length of piping during the life of the field to progressively adapt the injection pressure to the pressure of the field without creating fracturing.
No method exists, to date, for reducing pressure by between 0 and 50 bars without shear.